The present invention generally involves means and methods for measuring and/or monitoring parametric differences between associated fluid materials and is more particularly directed to measuring a pressure difference between fluids separated by a semi-permeable membrane. Pressure difference monitoring according to this invention presents a distinct advantage in extracorporeal blood systems, particularly in a procedure called therapeutic plasma exchange (TPE).
Many fluid systems require accurate measurements of various properties and/or parameters of the fluids flowing therethrough. In some of these systems, the importance derives from the measurements of individual parameters. In other cases, it is the change or difference in parameters that is important. In either event, the accuracy required for each particular fluid system may vary according to the particular fluid(s) involved and/or depending on the purpose of that system.
An example of a fluid system having special requirements which can be significantly impacted by the accuracy of parametric measurements, particularly involving pressure determinations, is a blood flow system outside the body, also known as an extracorporeal blood system. An extracorporeal blood system usually includes a device for processing the blood flowing therethrough. There are numerous types of such devices. Filtration devices having semi-permeable membranes are commonly used in extracorporeal blood systems such as those used in dialysis or therapeutic plasma exchange (TPE). The primary purpose of a semi-permeable membrane is usually to provide for the removal or separation of certain elements or components from the blood. Urea and other waste products are removed from blood in dialysis, and blood plasma is separated from the red blood cells in TPE. The processed blood or red blood cells are then returned to the patient.
More specifically, in an extracorporeal blood system using a semi-permeable membrane device, the process is as follows. Blood is removed from the patient, passed along and in contact with one side of a semi-permeable membrane. Unwanted portions of the blood (urea in dialysis, plasma in TPE) diffuse or filter through the pores of the semi-permeable membrane. The blood remaining on the blood side of the semi-permeable membrane is then returned to the patient with less of the unwanted substance.
Poor accuracy of pressure measurements in this art can create problems for the blood cells flowing through such a system. Excessive pressures or pressure differentials may cause red blood cells to become stuck in certain components of the system such as in the pores of a semi-permeable membrane and/or, at worst, these red cells may be pushed into or against certain system components until the red cells burst, a consequence called hemolysis. Repetitive red cell destruction in this fashion would then result in a reduction in the number of red blood cells available for carrying oxygen to the other cells of the body. A substantial reduction in red blood cells can thereby lead to oxygen deficiency injury or death. On the other hand, insufficient pressure differences in extracorporeal blood systems will result in less effective separation of the blood components from each other, as for example, of urea from the blood in a dialysis system, or of red blood cells from plasma in apheresis or therapeutic plasma exchange (TPE).
The performance of semi-permeable membrane systems, and indeed of the membranes themselves, depends, in part, on the pressure difference across the membrane which is called the trans-membrane pressure (TMP). Generally, as the TMP across the membrane increases, more unwanted substances pass through it. If the TMP on the membrane is large enough, the membrane will rupture or the blood will be damaged as described above. Therefore, there is often a desire to make the TMP as high as possible to make the treatment proceed faster, but not so high as to damage the membrane or the blood. The more accurately the TMP can be measured, the closer to the damage point the treatment can be performed.
Pressure difference monitoring across a semi-permeable membrane has been conventionally performed using two pressure transducers in the fluid system, one on each side of the membrane. Pressure readings are then taken and, either manually or using a microprocessor, one measured pressure is subtracted from the other. The resulting pressure difference is the trans-membrane pressure (TMP) referred to above. Also, because the fluid pressure varies along the length of the membrane, additional pressure transducers have also been used on either or both sides of a membrane to improve the accuracy of the ultimate TMP calculation. Average pressures on either or each side of the membrane can thus be obtained and these resulting average pressures subtracted one from the other to yield a better approximation of the actual pressure difference across the membrane.
More particularly, in conventional extracorporeal blood systems using a semi-permeable membrane disposed inside a filter device, it is common to measure the pressures outside, yet near the filter device with pressure transducers disposed adjacent the inlet and outlet of the filter device on the blood side of the membrane and adjacent the outlet of the filtrate side of the membrane. This allows calculation of an average TMP with the formula:       Average TMP    =                              Blood Inlet                +                  Blood Outlet                    2        -          Filtrate Outlet      
On the other hand, the maximum TMP experienced by the membrane needs only two of these transducer readings; namely, the pressure measurement at the blood inlet to the filter device and the measurement at the filtrate outlet. Thus, this maximum TMP maybe expressed as:
Maximum TMP=Blood Inletxe2x88x92Filtrate Outlet.
Thus, using three pressure transducers, one each at the blood inlet, blood outlet and filtrate outlet, both the average and maximum TMP""s can be calculated. Note, the semi-permeable membrane performance is generally associated with the average TMP, whereas failure of the membrane is usually related to the highest TMP experienced by the membrane.
Nonetheless, both of these (and all other conventional) methods also depend for accuracy upon the precision of the transducers used. And, most measuring systems have some inherent inaccuracy associated with them. Indeed, pressure transducers in this field commonly exhibit xc2x110% error in accuracy each relative to the actual pressure at that respective point in the fluid system. A linearity error of xc2x11% can also be expected. When using two or more of such transducers to determine a pressure difference, these error margins can then be compounded.
For example, in a typical pressure transducer system for an extracorporeal blood system which has an inaccuracy of xc2x110% for each transducer measurement, the overall accuracy of the pressure difference when measured with a two transducer system may be reduced by as much as a first xc2x110% from the first measurement. And, it may experience a still further accuracy reduction of an additional xc2x110% from the second measurement. This invention is intended to address this compounding of measurement error.
It is further apparent that there remains a distinct need for continued improvements in parametric monitoring particularly in fluid pressure difference evaluation which provides for more accurately determining the difference between the pressures occurring on both sides of a semi-permeable membrane. Better accuracy in pressure difference measurements will provide better achievement of target pressure differences in practice to substantially eliminate hemolysis and improve fluid component separation. It is toward all of these ends that the present invention is directed.
The present invention is directed to means and methods for approximating pressure differentials experienced in a fluid system. More particularly, the present invention involves using preliminarily measured and/or calculated correction quantities to modify the operationally measured pressure values to arrive at a closer approximation of the actual pressure differential.
In general, the correction quantities used herein are obtained by preliminarily pressurizing the system pressure transducers to various pre-selected pressures and recording the corresponding preliminarily measured values for each transducer in a data table for later use as or in the derivation of correction quantities. A first use of such correction quantities is to interpolate between the two closest data table values relative to the operationally measured pressure value and use the resulting interpolated value as the corrected pressure value. This sort of interpolation may be performed for each of two pressure transducers, one on each side of the membrane. The resulting corrected pressure values are then subtracted from each other to obtain the corrected pressure difference or TMP. An alternative of this correction scheme involves using data table correction quantities of a reference pressure transducer in the interpolation calculations of the two membrane pressure transducers.
Similarly, other correction quantities can be recorded in a data table during a preliminary pressurization phase as describe briefly above. For example, the respective differences between the two preliminarily measured pressures of each of the trans-membrane pressure transducers may be recorded as correction quantities for each preliminarily applied pressurization. These correction quantities can then be used to mathematically modify the operationally measured pressure differential during actual fluid flowing use. Also, a reference transducer can be used here as well such that the differences between one membrane transducer and the reference transducer can be recorded in the data table as one set of correction quantities, and the differences between the other membrane transducer and the reference can be recorded as a second set of correction quantities. Both correction quantities may then be used in the ultimate determination of the pressure difference across the membrane, the TMP.
Other fluid parameters such as temperature, volume, flowrate and the like can also be better evaluated according to the present invention. For the purposes hereof, fluids include gases and/or liquids.
Accordingly, the primary object of the present invention is to provide improved accuracy in determining the parameters exhibited in a fluid system, particularly in determining pressure differentials in fluid systems having two or more fluids separated by a membrane.
A further object is to improve pressure differential accuracy using only two pressure transducers; one on each side of a membrane.
A still further object is to improve pressure differential accuracy using two pressure transducers; one on each side of a membrane both modified relative to a third pressure transducer.